# std::throw_with_nested

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Defined in header `<exception>` |  |
template< class T > [[noreturn]] void throw_with_nested( T&& t ); |  | (since C++11) (constexpr since C++26)

If std::decay<T>::type is a non-final non-union class type that is neither std::nested_exception nor derived from std::nested_exception, throws an exception of an unspecified type that is publicly derived from both std::nested_exception and from std::decay<T>::type, and constructed from std::forward<T>(t). The default constructor of the `nested_exception` base class calls std::current_exception, capturing the currently handled exception object, if any, in a std::exception_ptr.

Otherwise, throws std::forward<T>(t).

Requires that std::decay<T>::type is CopyConstructible.

### Parameters

t | - | the exception object to throw

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_exceptions` | `202411L` | (C++26) | constexpr for exception types

### Example

Demonstrates construction and recursion through a nested exception object.

Run this code

    #include <exception>
    #include <fstream>
    #include <iostream>
    #include <stdexcept>
    #include <string>

    // prints the explanatory string of an exception. If the exception is nested,
    // recurses to print the explanatory string of the exception it holds
    void print_exception(const std::exception& e, int level =  0)
    {
        std::cerr << std::string(level, ' ') << "exception: " << e.what() << '\n';
        try
        {
            std::rethrow_if_nested(e);
        }
        catch (const std::exception& nestedException)
        {
            print_exception(nestedException, level + 1);
        }
        catch (...) {}
    }

    // sample function that catches an exception and wraps it in a nested exception
    void open_file(const std::string& s)
    {
        try
        {
            std::ifstream file(s);
            file.exceptions(std::ios_base::failbit);
        }
        catch (...)
        {
            std::throw_with_nested(std::runtime_error("Couldn't open " + s));
        }
    }

    // sample function that catches an exception and wraps it in a nested exception
    void run()
    {
        try
        {
            open_file("nonexistent.file");
        }
        catch (...)
        {
            std::throw_with_nested(std::runtime_error("run() failed"));
        }
    }

    // runs the sample function above and prints the caught exception
    int main()
    {
        try
        {
            run();
        }
        catch (const std::exception& e)
        {
            print_exception(e);
        }
    }

Possible output:

    exception: run() failed
     exception: Couldn't open nonexistent.file
      exception: basic_ios::clear

### See also

nested_exception (C++11) | a mixin type to capture and store current exceptions (class)
rethrow_if_nested (C++11) | throws the exception from a std::nested_exception (function template)

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